Urticaria and angioedema
- Lesion
Hives are itchy wheals that come and go within hours. Mast cells release histamine, which makes small vessels leak.
What it is
Urticaria is a localised, transient dermal oedema caused by mast cell degranulation. Individual wheals usually fade within a day, and the episode is acute if it lasts under six weeks and chronic if longer. Angioedema is the same process deeper in the dermis and subcutis; when it occurs without wheals, bradykinin-mediated causes such as C1 inhibitor deficiency or ACE inhibitors must be considered.
Step by step
- A trigger activates mast cells (IgE cross-linking or a direct stimulus).
- Granules release histamine, and the cell makes prostaglandin D2 and leukotrienes.
- Histamine on H1 receptors makes venules leak and nerves itch.
- Plasma leaks into the superficial dermis (wheal) and the reflex axon dilates nearby vessels (flare).
- Fluid is reabsorbed within hours, so the wheal disappears without a mark.
Clinical notes
- Wheals lasting more than a day, burning rather than itch, or residual bruising suggest urticarial vasculitis.
- Angioedema of the larynx can obstruct the airway.
Test yourself
Think of the answer, then tap the question to see it.
What is seen on biopsy of ordinary urticaria?
Superficial dermal oedema, dilated lymphatics and sparse perivascular lymphocytes and eosinophils; the epidermis is normal.
Why does hereditary angioedema not respond to antihistamines?
It is mediated by bradykinin, from uncontrolled kallikrein activity due to C1 inhibitor deficiency, not by histamine.
What are two IgE-independent triggers of urticaria?
Opioids or radiocontrast (direct mast cell activation) and NSAIDs; also physical triggers such as cold or pressure.
Elsewhere in DurgiPedia
- Type I hypersensitivity (immediate, IgE)
- Mast cell and basophil
- Histamine, serotonin, bradykinin and the plasma protein cascades
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Hives (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.